The dossier on Proxima Centauri reads something like this: Closest star to the Sun, at 4.2 light-years. A cool red dwarf, just one-eighth as massive as the Sun, and one ten-thousandth as bright. Covered with dark "starspots," and produces giant eruptions that saturate space with radiation. No known planets.

Astronomers keep trying to fill in that blank space in the "planets" category, but they just haven't found anything. They've used Hubble Space Telescope to look for a "wobble" in the star's position caused by the gravitational tug of a planet. They've looked for planets to pass in front of the star, dimming its light a little. And two astronomers spent years looking for the signature of a planet in the star's spectrum -- the individual wavelengths of its light.

That search was conducted by Texas astronomer Mike Endl and a colleague, Martin Kurster. In 2008, they reported that their search had turned up empty. The search was sensitive enough to detect a planet just a few times the mass of Earth in the star's "habitable zone" -- the distance away from the star where temperatures are just right for liquid water. Because Proxima Centauri is so small and faint, its habitable zone is much closer in than Earth is to the Sun.

It's still possible that planets could circle Proxima Centauri. But they'd have to be pretty small or pretty far out to have eluded the searches for worlds around our closest neighbor.